This proposal is based upon three eye observations: 1) We have previously shown that anti-B7-1 antibody administration will block experimental autoimmune encephalomyelitis (EAE) in SJL mice, however, our recent results show that B7-1 deficient mice on the SJL background are more suitable to EAE than wild type SJL mice; 2) We have found that whereas B7-1/B7-2 double deficient mice continue to be susceptible on the SJL background. Thus, the requirements for co- stimulation in the induction of EAE on different genetic backgrounds are different. Whether this is due to genetic differences, or differences in the encephalitogenic linked to a locus on chromosome 1 which encompasses the CD28, CTLA4, ICOS genes. Whether these genes or others within this locus are determinants of this susceptibility need to be determined. Using mice with targeted deficiencies of these co-stimulatory pathways, and congenic NOD mice in which the susceptible CD28/CTLA4/ICOS locus has been replaced with the resistant allele, we propose to define mechanisms by which these co-stimulatory receptors contribute to genetic susceptibility/resistance to autoimmunity. Three specific aims ha these questions have been proposed to address these questions: i) To investigate whether the co-stimulatory molecules B7-1 and B7-2 play a role in the selection and expansion of the autoreactive repertoire by determining whether B7-1 and/or B7-2 play important roles in central tolerance and whether the elimination of B7-1 and/or B7-2 results in the escape of PLP 139-151 reactive T cells and an increase the size of the autoreactive T cell repertoire in the periphery, resulting in enhanced autoimmune disease. ii) To mice on the SJL and B5 backgrounds by determining a) whether the difference in susceptibility on the two backgrounds is due to different repertoires to the antigens used or b) whether the genetic background modifies the need for co-stimulation for the induction of EAE. iii) Study the role of the CD28/CTLA4/ICOS locus in genetic susceptibility and resistance to EAE and diabetes by using congenic and subcongenic NOD mice to susceptibility and b) study the cellular mechanism(s) of resistance by studying the proliferation and cytokine phenotype of the responding T cells from the congenic mice. These studies will provide fundamental information on the role of the CD28/CTLA4/ICOS co-stimulatory pathway in the induction, regulation and genetic resistance to two autoimmune diseases, EAE and diabetes.